linux-kernel - Re: [PATCH v1 2/2] perf auxtrace: Optimize barriers with load-acquire and store-release

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Message-ID: <YLX7OhK37QmfeOwu@hirez.programming.kicks-ass.net>
Date:   Tue, 1 Jun 2021 11:17:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] perf auxtrace: Optimize barriers with
 load-acquire and store-release

On Tue, Jun 01, 2021 at 12:07:31PM +0300, Adrian Hunter wrote:
> __sync_xxx_compare_and_swap is out-of-date now. This page:
> 
> https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins
> 
> recommends '__atomic' builtins instead.

perf doesn't seem to use that.

> Since atomics are needed only for the "compat" case (i.e. 32-bit perf with 64-bit kernel)
> you could try to find an elegant way to check for a 64-bit kernel, and avoid the atomics
> for a 32-bit perf with 32-bit kernel.

Most 32bit archs cannot do 64bit atomics. I suppose the only reason this
doesn't explode is because the aux stuff isn't supported on many
architectures?